Method of caching digital certificate revocation lists

ABSTRACT

A method of updating a locally cached revocation list of digital certificates in a client computer with a current data base of revoked certificates stored in a host computer. Upon receipt of a digital certificate submitted to the client computer for verification, the client computer checks the submitted digital certificate against the locally cached revocation list. If the submitted digital certificate is not on the locally cached revocation list, the client computer confirms the validity of the submitted digital certificate and the transaction is consummated. If however, the submitted digital certificate is on the revocation list, the client computer establishes a communication link with the host computer and determines if the submitted digital certificate is in the data base of revoked certificates on the host computer. Without breaking the communication link, the client may request that an updated revocation list to be downloaded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 08/528,912, filed on Sep. 15, 1995, by David W. Aucsmith, now U.S. Pat. No. 5,701,464, entitled PARAMETERIZED BLOOM FILTERS and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for determining membership in a database. More specifically, the present invention relates to the use of parameterized bloom filters in order to determine membership in database, such as a key invalidity database, to determine key validity.

2. Background Information

The use of credit cards, debit cards, or other types of non-cash payment methods for a transaction requires a determination of the validity of the payment means prior to the completion of the transaction. Current methods of determining validity of such payment methods includes a client/server system such as 100 shown in FIG. 1. In one prior art method, a payment method, such as a credit card 112, is used for inputting a key 114 into a client 110. Because the list of revoked credit card numbers or "keys" 114 is very large, the client 110 performs a query 116 of key validity, for example, to a server 120. Server 120 has access to a very large invalidity database 121 which contains a list of all of the invalid keys. Thus, client 110 makes a query 116 to server 120 whether the key 114 is in the database 121. The database could contain all keys, valid and invalid, and mark which keys are invalid. For simplicity, it is assumed that only invalid keys will be found when the database is querred.

Responsive thereto, server 120 performs a lookup in an invalidity database 121 to determine the presence of the key. If the key is present in the database, then it is invalid. Server 120 then either provides a valid or invalid signal to client 110 indicating whether the key is a member of the invalidity database 121 or not. If a valid signal is returned (indicating the key is not in invalidity database 121) client 110 completes the transaction.

One shortcoming of some prior art database membership applications, such as those used in the credit card verification application discussed above, is that membership in the database is determined based upon of a request from a client and a corresponding response from a server. That is, for every transaction, a communication link must be established and a database lookup must be performed. Because keys, such as credit cards, have large numbers of possible values, and correspondingly large numbers of possible invalid keys, the server 120 is capable of communicating with large numbers of clients and can service large numbers of requests.

In order to respond to all the possible requests which may occur, especially at high traffic intervals, server 120 requires a large number of communication channels, and typically should be able to service a large number of search requests in invalidity database 121 efficiently. This requires a high performance server, which may increase costs dramatically, according to the implementation.

Thus, the prior art has suffered from the shortcoming that very expensive mechanisms for verifying the validity of keys, and correspondingly large amount of communication bandwidth must be provided in order for such a key validity determination system to be fully practicable. Such a capacity stretches resources both in terms of communication resources, computational resources, and the corresponding cost of providing such resources.

Thus, a need exists for a method and apparatus which can determine membership in a database, such as an invalidity database, but yet, minimizes computational, communication, and cost resources.

SUMMARY OF THE INVENTION

Briefly, one embodiment of the invention includes a system comprising a memory containing a condensed revocation list and circuitry for retrieving an update version of the condensed revocation list.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitation in the figures of the accompanying in which like references indicate like elements and in which:

FIG. 1 shows a prior art validity determination system.

FIG. 2 shows an invalidity detection system.

FIG. 3 shows a computer system in which either a client and/or a server may be implemented.

FIG. 4 shows the format of a data string which is returned from a server for a current bloom filter to be stored in the client.

FIG. 5 shows a functional block diagram of processes or dedicated circuitry which may be contained within a client for providing the function of a local invalidity cache of an invalidity database.

FIG. 6 shows a functional block diagram of processes or circuitry contained within a server which provides membership confirmation signal(s), bloom filter coefficients and vectors to a client upon request.

FIG. 7 shows a process which is performed for key invalidity detection within a client.

FIG. 8 shows a process performed by a client for negotiation with a server to obtain a current bloom filter string.

FIGS. 9a-9c show a series of messages which may be transmitted between a client and a server in order to make available a current bloom filter to a requesting client.

FIG. 10 shows a sequence of steps performed within a server for updating a bloom filter matrix which is used for providing bloom filters to requesting client(s).

FIG. 11 shows a sequence of steps performed within a client for determining whether a key is in the local invalidity cache.

FIGS. 12a-12c shows details of the process performed within a server for calculating the bloom filter matrix.

DETAILED DESCRIPTION

Embodiments of the present invention include an invalidity detection system wherein a client maintains a local invalidity cache of a database stored in a remote system, such as a server. The client connects with the server only if a key is potentially a member of an invalidity database, as indicated by the local invalidity cache. The local invalidity cache of the invalidity database, is a hash function of all invalid keys. If the key does not exist in the local invalidity cache, then it is guaranteed that it does not appear in the invalidity database. In the instance where the key may be invalid, that is, it may be a member of the invalidity database, then the client can request that the server perform a lookup in an invalidity database. Although the present invention will be described specifically with reference to certain specific embodiments, especially those shown in FIGS. 2-12c, these embodiments are described for the purposes of illustration only, and do not limit the present invention. Modifications and other changes may be performed without departing from the overall spirit and scope of the present invention.

Overview

FIG. 2 illustrates a system which is used for invalidity detection of keys in implemented embodiments of the present invention. A client system 210 accepts a key, a unique value 214, for example, the number of a credit card 212. The database 222 may include a list of revoked credit card numbers, ID badge numbers, or other unique values which are no longer valid. The client determines whether the key is in invalidity database 222 to determine whether a transaction will be allowed to proceed.

Upon receipt of a key 214 the client 210 performs a lookup in a local invalidity cache 218. A response signal indicating the results of the lookup in the local invalidity cache 218 is now generated. As will be described, local invalidity cache 218 comprises a bloom filter which is generated by performing one or more hash functions of the invalid keys contained in the invalidity database 222 stored in the server. If a lookup in the local invalidity cache 218 indicates that the key is not a member of invalidity database 222, then the key is guaranteed to not be a member of invalidity database 222. In this case, no communication between the client 210 and server 200 need take place.

If the local invalidity cache 218 indicates that the key is a member of invalidity database 222, then this may or may not be true. In that instance, the client may perform an invalidity confirmation process wherein the client communicates with the server 220 and requests a lookup to confirm the key's membership in invalidity database 222. No invalidity confirmation process need take place if the client can accept the possibility of the key actually being valid, within an error measure "a". In the case of a database lookup, server 220 provides signals 230 to client 210 indicating whether the key is or is not a member of database 222. There is a possibility that a finding of membership in the local invalidity cache by the client will result in membership not being found in invalidity database 222, however, the error a is calculable and is relatively small. The error "a" may be used by the client to determine the local invalidity cache's accuracy and may determine whether a lookup is even required in some implementations.

Local invalidity cache 218 is a bloom filter of all the invalid keys in the database 222. Server 220 includes a bloom filter/matrix calculator 224 which generates bloom filters for various parameters at periodic intervals from invalidity database 222. The bloom filter and matrix calculator 224 accesses invalidity database 222, and hashes all keys contained within the database for a range of parameters, error values, number of coefficients of the hash functions, and a prime key value which is used during the process for determination of whether the key is a member of the local invalidity cache 218. Any of the bloom filter vectors, coefficients, and prime key values may be downloaded from server 220 by client 210 upon demand, at regular intervals, or other defined intervals according to the established protocol. In implemented embodiments, the bloom filter vectors, coefficients and parameters stored in the matrix 224 may be computed at some interval, such as the beginning of the business day or when the invalidity database 222 is updated by some threshold measure.

Computer System

Referring to FIG. 3, a system 310 upon which one embodiment of a computer system (e.g. client 210 or server 220) of the present invention as implemented is shown. System 310 comprises a bus or other communication means 301 for communicating information, and a processing means 302 coupled with bus 301 for processing information. System 310 further comprises a random access memory (RAM) or other volatile storage device 304 (referred to as main memory), coupled to bus 301 for storing information and instructions to be executed by processor 302. Main memory 304 also may be used for storing temporary variables or other intermediate information during execution of instructions by processor 302. System 310 also comprises a read only memory (ROM) and/or other static storage device 306 coupled to bus 301 for storing static information and instructions for processor 302, and a data storage device 307 such as a magnetic disk or optical disk and its corresponding disk drive. Data storage device 307 is coupled to bus 301 for storing information and instructions.

System 310 may further be coupled to a display device 321, such as a cathode ray tube (CRT) or liquid crystal display (LCD) coupled to bus 301 for displaying information to a computer user. An alphanumeric input device 322, including an alphanumeric and other keys, may also be coupled to bus 301 for communicating information and command selections to processor 302. An additional user input device is cursor control 323, such as a mouse, trackball, stylus, or cursor direction keys, coupled to bus 301 for communicating direction information and command selections to processor 302, and for controlling cursor movement on display 321.

In implemented embodiments, another device which may be coupled to bus 301 includes a serial interface 324. This may be coupled to a key input device 326 which can automatically input a key. This type of device may include a bar code, OCR (Optical Character Recognition), magnetic reader or other type of automatic key input device.

System 310 may also include a communication device 325 which comprises a means for communicating with other devices. This communication device may also include a means for communicating with other nodes in a network or across telephone lines. In implemented embodiments, this may include a MOdulator/DEModulator (MODEM) for communication between client 210 and server 220. Note, also, that any or all of the components of system 310 and associated hardware may be used in various embodiments, however, it can be appreciated that any configuration of the system that includes a processor 302 may be used for various proposes according to the particular implementation.

In one embodiment of a client 210, system 310 my be one of the IBM AT-compatible type personal computers such as the Gateway 2000 brand personal computer manufactured by Gateway Computer Systems. Processor 302 may be one of the Pentium" brand microprocessors available from Intel Corporation of Santa Clara, Calif. (Pentium and Intel are trademarks of Intel Corporation).

Note that the following discussion of various embodiments discussed herein will refer specifically to a series of routines which are generated in a high-level programming language (e.g., the C or C++ language) and compiled, linked, and then run as object code in system 310 during run-time. It can be appreciated by one skilled in the art, however, that the following methods and apparatus may be implemented in special purpose hardware devices, such as discrete logic devices, large scale integrated circuits (LSI's) application-specific integrated circuits (ASIC's), or other specialized hardware. The description here has equal application to apparatus having similar function.

The Invalidity Database

Database 222 contains N items. Each item in the database is uniquely identified by a key Q so that there are Q₁ . . . Q_(w) . . . Q_(N). Each key, Q_(w), consists of t bytes q such that Q_(w) =q_(w1) . . . q_(wi) . . . q_(wt). A bloom filter is used in implemented embodiments for the local invalidity cache 218. No search in invalidity database 222 need be performed if the key is not present in local invalidity cache 218. Database 222 may be, for example, a database of revoked credit cards. A bit vector, B, is maintained in the local invalidity cache 218, so that most keys not in the file can be recognized as absent without making any references to database 222.

An internal bit vector, B, known as a Bloom vector is stored in client 210 with bits b₀ b₁ . . . b_(M-1), where M is rather large. During creation of the Bloom Vector by the server 220, each bit b_(x) of B is initially set equal to 0. For each key Q_(w) in the file, k independent hash functions, H₁ (Q_(w)) . . . H_(j) (Q_(w)) . . . H_(k) (Q_(w)) are computed by the server and the corresponding k b's are set equal to 1 in the bloom vector. (These k values need not be distinct.) Thus b_(x) =1 iff H_(j) (Q_(w))=x for some j and w.

To determine if a search argument Q is in the external database using the Bloom vector, client 210 first tests whether or not b_(x) is equal to 1 for H_(j) (Q)=x. If it is not, then Q is guaranteed to not be in the database 222. If it is set equal to 1 then each of the k hash functions are continued to be tested. If all k hash functions correspond with bits equal to 1 then Q is in the external database 222 with a certain probability 1-a. The probability of a "False positive" a when there are N records in the database 222 is: ##EQU1##

Some prior art systems employ bloom filters which are static storage devices with k and M fixed and with N varying. In implemented embodiments of the present invention, N is fixed at any given time. That is, there are N members in the database 222. Embodiments provide a method for constructing a Bloom filter B encoding the N members of the database 222 in a way that allows a, k, and M to be chosen to optimize the vector B for client 210's needs.

In one embodiment of the present invention a function is used:

    Γ(x)=p

where p is the next prime number greater than the number x. This may be accomplished using any number of techniques including Euclid's or Verasmus' theorems. The Bloom vector B is modified so that the calculated or chosen value M is replaced by

    Γ(M)=p.sub.M

so that B is a vector of p_(M) bits. A set of k hash functions, H₁ (Q_(w)) . . . H_(j) (Q_(w)) . . . H_(k) (Q_(w)) are used by the sever 220 that map uniformly over B where ##EQU2## The function H_(j) (Q_(w)) maps uniformly over B as long as ##EQU3## Constructing hash functions in this manner allows for an arbitrary number of hash functions. Thus, k, M, and a can be varied for a given N.

Client 210 uses the bloom vector B to verify membership of item Q in database 222 with the client 210 off-line from the server 220. The server 220 varies the number of hash functions H(Q) in order to maintain the Bloom filter's efficiency. An arbitrary number of hash functions is stored in a matrix and the client 210 and server 220 negotiate the Bloom filter parameters, k, M, and a for a given number of invalid keys N.

Server 210 maintains the database 222 of items (e.g. invalid credit card numbers). For a given value of N, the server computes an optimized Bloom vector for each parameter of interest. Thus, there are Z bloom vectors B₁ . . . B_(r) . . . B_(z). The server then stores each of these Bloom vectors in a data string 400 as shown in FIG. 4. These are accessible via a bloom filter matrix addressable by the three parameters.

The Bloom Filter String

Bloom vector strings are downloaded by a client from a server to determine the possible database membership of a key. A bloom filter string 400 illustrated in FIG. 4 has a serial number 402 which is a unique sequential identifier for the data string. This number may be used by a client to determine if it has the most up-to-date vector. The Date and Time fields 404 and 406 containing the date and time of the vector's creation for a similar purpose. The error value "a" in field 410 is the probability of a false positive when using the vector to test a key. The values N and k stored in fields 412 and 414 are the other parameters used in the vector's construction.

The hash function ##EQU4## can be rewritten in the form:

    p.sub.j ∫(Q.sub.w)modp.sub.M

where ##EQU5## Thus, the values p₁ . . . p_(j) . . . p_(k) contained in field 416 correspond to the p_(j) for a given k and p_(M). The remaining portion of the data string is field 418 containing the prime key p_(M) and the bits, b₀ b₁ . . . b_(pM-1) in field 420 that constitute the actual vector. The server may also create multiple Bloom vectors, B, based on any other combination of parameters such as those for a given k.

Client Functional Block Diagram

FIG. 5 illustrates a functional block diagram of processes which are present in client 210 for performing the functions of the local invalidity cache 218. Invalidity cache 218 in FIG. 5 may include processes which are resident within memory of the client during the client's runtime, or alternatively, be dedicated circuitry such as firmware, or discrete circuitry for performing the described functions.

Local invalidity cache 218 includes an invalidity detector 502 which receives as an input a key and provides as an output a valid/invalid signals to a user. The invalidity detector 502 communicates with a control circuit 504, which interacts with the parameterized bloom filter 506. If circuit 506 indicates that the key is possibly a member of the database, via the assertion of an invalid signal, then control 504 activates confirmation module 510 which interacts with server 220 to request a database lookup within database 222. The key and request signals are sent over communication link 230 to server 220.

Bloom filter 506 further communicates with parameters filter request module 508 which requests and obtains the coefficients and bloom vectors from the server 220. Module 508 negotiates with server 220 in order to provide the client-required parameters of the bloom filter to the server, and receive the bloom filter string 400.

The desired parameters, if any, a, k, and M are provided by client parameters circuit 512 which receives and stores the time/space requirements configured within client 210. These are also provided to the parameter/filter request module 508. Module 508 may be operative at various periodic intervals, including when the bloom filter contained within bloom filter 506 becomes out of date, and a request for a key lookup in the invalidity cache 218 is performed in client 210. Bloom filters may also be updated at other periodic intervals such as the beginning of a business day, or when the serial number and/or date of the bloom filter changes. Other periodic intervals may be used for updating of the bloom filter in different implementations.

Server Functional Block Diagram

FIG. 6 illustrates a functional block diagram of bloom filter calculation circuit/matrix 224 in server 220 illustrated in FIG. 2. These may also be processes resident in computer system memory of server 220. Module 224 includes a control module 602 which receives parameters, keys, request signals to and provides coefficients, vectors and other information from the server 220 over communication medium 230. According to operating circumstances, control module 602 controls and communicates with the bloom filter matrix 604, the bloom vector calculator 606 and lookup module 608. In the event of a lookup request from a client, control circuit 602 causes lookup module 608 to access database 222. Responsive thereto, the results, whether the key is present or not present in database 222, is then provided to client 210.

In the event of a request of an update of the bloom filter contained within the client, the control module 602 communicates with bloom filter matrix 604 and, according to the parameters provided to the server, accesses the appropriate bloom filter string containing the client-specified parameters, a, k, and M, as well as the coefficients p₁ . . . p_(k) of the hash functions, the bloom vector B and the prime key p_(M). Control module 602 negotiates with client 210 in order to obtain the requested bloom filter string according to the client's time/space requirements.

Circuit 224 further includes the bloom vector calculator 606. Bloom vector calculator 606 is operative at periodic intervals, when control module 602 determines that a new bloom filter matrix 604 is desired to be calculated. This may occur at various periodic intervals, such as once a business day, upon a threshold increase in the number of invalid keys contained within database 222, or some other update criteria. Bloom vector calculator 606 accesses invalidity database 222, and the values of M, k, and a and calculates the coefficients and the vectors of the bloom filter string for the various parameters. These are then placed in the bloom filter matrix 604 for accessing of bloom filter string by clients upon request.

Client Processing

The process performed within the client for key invalidity detection is shown as 700 in FIG. 7. For the purposes of this embodiment, the client awaits input at step 702 of a new key which the user wishes to check for invalidity at step 702. Once a key is input, as detected at step 702, it is determined at step 704 whether the bloom filter string stored in the client is the current bloom filter string. This may be performed in any number of manners, including but not limited to, the checking of the serial number against a serial number broadcast by the server, the checking of a date if updates of the bloom filter are performed on a daily basis, or other pre-established protocol between the client 210 and the server 220.

If the current bloom filter has not been stored in the client, as detected at step 704, then at step 706, the client negotiates with the server to obtain the current bloom filter. This process may include specification by the server of the range of parameters that is capable of providing bloom filter coefficients and vectors for, as well as step values indicating the step size between available parameters within the range. Other negotiation protocols may be performed. Once the new bloom vectors have been retrieved, because the connection has already been established with server 220, a key lookup may be requested to be performed by server 220 at step 714.

If the current bloom filter string has been stored in the client 210 then, at step 708, the key is checked to determine if it is present in the local invalidity cache 218 at step 708. The determination of whether the key is in the local invalidity cache is performed using the coefficients of the hash functions, p₁ . . . p_(k), the prime key p_(M), and the bloom vector b₀ b₁ . . . b_(pM-1). The details of the determination of whether the key is in the local invalidity cache is discussed in more detail with reference to FIG. 11.

Subsequent thereto, if the key is detected as being present in the local invalidity cache as detected at step 712 then the client requests a lookup of the key in the invalidity database 222 by server 220 at step 714. If the key is not in the local invalidity cache, then the key is indicated as valid to the user at step 720, and the process returns to step 702 for the detection of the input of additional keys. Thus, in the instance where the key is not in the local invalidity cache 218, it is guaranteed that the key is not in the invalidity database 222, and no communication between the client 210 and the server 220 need take place.

In the event that the local invalidity cache indicates that the key is present, there is still a probability a that the key may not be present in the invalidity database 222. Thus, a key lookup must be performed at step 714 in the server 220 in order to ensure that the key is a member of invalidity database 222. In some implementations, no such lookup may be required to take place if the error a is acceptable. At step 714, a key lookup request is sent to the server 220 with the key in a request signal to server 220 by the client 210. Then, at step 716, it is determined whether the server indicates whether the key is valid or not. That is, if the key is indicated as valid, then the key was not present in invalidity database 222. The user is then notified at step 720 that the key is valid. If, however, the server indicates that the key is invalid as detected at step 716, then the user is alerted at step 718 that the key is invalid. The key is a member of invalidity database 222.

FIG. 8 illustrates the process 706 in more detail which shows the negotiation process from the client's perspective for the current bloom filter string. Although 706 was shown within the key invalidity detection process 700, it can be appreciated by one skilled in the art that this may be performed at any periodic interval, such as a predetermined time or at daily intervals. However, the embedding of step 706 within a key invalidity detection process 700 above optimizes the transfer of the bloom filter strings between the server and the client to time periods in which actual key lookups are required to be performed in the client 210. Thus, the bloom filter will be updated when communication links are otherwise already established.

Process 706 starts at step 802 wherein the client requests the range of parameters supported by the server. At step 804, the client receives the available parameters from the server. For example, FIG. 9a illustrates the receipt of the range of parameters for k and M including k_(MAX) or M_(MAX) (the maximum values of M and k), a_(MIN), (the minimum error value), and step sizes for any of these values (e.g. k_(STEP), M_(STEP), a_(STEP)). Once these values are received by the client, the client can select the parameters of interest, if any are desired, at step 806. This is illustrated in FIG. 9b, wherein the relevant parameters k, M, and a are transmitted 910 to server 220 by client 210. If any parameters are not specified (at least two are required to obtain a unique bloom filter), then appropriate default values may be used by the server for providing the bloom vectors to the client.

At step 808, the client requests the appropriate bloom filter string with the specified parameter(s), if any. At step 810, the client then receives the bloom filter string 920, as illustrated in FIG. 9c. The local invalidity cache 218 is thus updated.

Bloom Vector Calculator

FIG. 10 illustrates the process 1000 which may be performed by bloom vector calculator 606 in FIG. 6 at periodic intervals when a new bloom filter is required to be computed in server 220. This process is thus performed for the entire range of M's, k's and a's which the server supports. Process 1000 is performed at a periodic interval, such as a daily invalidity database update time period. Thus, all computation may be performed off-line by the server and the new bloom filter strings are made available to clients upon request. Process 1000 commences at step 1002 and determines whether a new set of bloom filters is required or not. Whether a new set of bloom filters is required may be determined based upon any of a number of circumstances, including a threshold number of new invalid keys, a daily bloom filter update period or other implementation-specific determination. Then, at step 1004, the range of parameters is determined. At step 1006, the bloom filter matrix is calculated based upon the current invalid keys and the range of parameters which are desired to be maintained by the server. In this implementation, subsequent to the calculation of the bloom filter matrix, the server may broadcast the current date and serial number of the current bloom filter strings available to potential clients of the server. Other embodiments of this invention may also be performed wherein clients are notified on a regular basis of the new serial number and date of the bloom filters, upon an initial connection with a client to determine whether the client's serial number and/or date is out of date, or other implementation-specific protocol. Upon completion of step 1008, process 1000 is thus complete.

Local Invalidity Cache

FIG. 11 illustrates the details of process 708 which determines whether the key is in the local invalidity cache 218. Process starts at step 1102 which initializes the variables i and Q_(TOT). Then, at step 1104, the product of all the t bytes ^(q) w_(i) of the key Q_(w) are accumulated in the Q_(TOT) value. It is then detected at step 1106 whether i is yet equal to t, the total number of bytes contained within the key. If not, i is incremented at step 1108, and steps 1104-1108 continue until the product of all the t bytes ^(q) w_(i) in the key have been accumulated in Q_(TOT).

Once all of the t bytes have been accumulated in Q_(TOT) then, at step 1110, a modulo (MOD) is performed of the accumulated Q_(TOT) value using the prime key p_(M). This is performed at step 1110. Continuing at step 1112, the index j is initialized to 1. This will then allow the checking of all of the bytes in the bloom vector B against the product of each of the parameters and the Q_(TOT) value. At step 1114 the variable TEST is set equal to the product of the current parameter p_(j) and Q_(TOT). It is then determined at step 1116 whether TEST has the same bit pattern as bloom vector B. If not, then the process returns at step 1124 indicating that the key is not present in the local invalidity cache 218, and thus it is not present in the invalidity database 222. It is therefore guaranteed that the key is valid--it is not a member of invalidity database 222. No communication with server 220 need take place.

Step 1116 is a bitwise comparison of the bits set in the variable TEST and corresponding bits in the bloom vector B. Thus, the step 1116 comprises testing for each set bit in the variable TEST whether the corresponding bit in the bloom vector B is set. If not, then step 1116 yields a negative result--TEST is not equal to B. If the bit is set, then the next set bit in TEST is checked against the corresponding bit in B, and this pattern continues until all of the set bits in TEST have been compared against the corresponding bits in B. Thus, this further improves the efficiency of the local invalidity cache because all bits in each hashed key value stored in TEST do not have to be checked if one is not set in the corresponding bloom vector B. In addition, no additional coefficients of the hash functions need to be applied to the key because one corresponding unset bit in the bloom vector B also causes the test to fail for all remaining untested hash function coefficients.

In the event that TEST has the same bit pattern as bloom vector B, as detected at step 1116, it is then detected at step 1118 whether all the coefficients p_(k) have been checked, that is, that j=k. If j is not equal to k, then j is incremented at step 1120, and steps 1114-1118 repeat. This process continues iteratively until all of the coefficients p_(k) have been checked or TEST is not equal to B. Once all of the parameters have been checked to determine whether the bits set in the bloom vector B are equal to the bit pattern p_(j) *Q_(TOT) then j=k. In that event, process 708 then proceeds to step 1122 and the process returns indicating that the key may be present in invalidity database 222. A confirmation via a lookup in server database 222 may be performed if required in the implementation. No database lookup need be performed in some implementations, as long as the error a is acceptable.

Process 708 is thus more efficient than prior art client/server techniques which always require a server lookup. Although some computation is required to be performed by the client, the advantage of conservation of communication resources, especially during high traffic intervals, outweighs any disadvantages posed by some local computation in the client.

In early electronic certificate systems, the creator or the certificates pushes the revocation list to the users. That is, the creator of the certificates supplies the revocation list at its own discretion. The user/client has no voice in determining how often it may receive updated revocation lists.

For example, in the old credit card environment, a credit card was valid if (1) it had not expired and (2) it was not on the credit card issuer's revocation list. Credit cards that became invalid because they had expired could easily be checked because the expiration date is on the credit card. However the revocation lists created a different problem since it changed continually. As a partial answer to this problem, a written list of revoked credit cards was mailed periodically (weekly, monthly) to each merchant. If a credit card were invalid, the credit card company would not honor a purchase made with that card. The merchant had the responsibility of checking the list before consummating each sale.

The current practice among sophisticated merchants is to go on line with each transaction to check the revocation list. That is in the typical case, prior to consummating each transaction, a client computer with a modem makes a telephone line connection with a server/host computer and checks the invalidity data base resident in the server. There is only one revocation list; it is on line; and it is always current. The push of a revocation list from the issuer to the merchant is not necessary. This system is of course works well but it is expensive. And because it is expensive, it will not be suitable for many electronic transactions of the future, for example the purchase of a soft drink from a vending machine.

Thus, with the present invention, however, it is assumed that on line access to a current revocation list is not economically feasible for each transaction. However, it is economic to go in line occasionally. In this environment, revocation lists in the form of parameterized Bloom filters are stored locally in the client. The client can now decide based on its own economics when it wishes to have its revocation list updated. Thus, the revocation list may now be "pulled" from the server to the client. The factors that will most likely determine the frequency of such a pull by the client is the communication link connection cost, the risk and expense of turning away a valid customer and the value of the transaction.

With the present invention, however, a particular situation is set up where the pulling of an updated revocation list is likely to be economic and therefore desirable. With each transaction at the client, the revocation list is interrogated by hashing. If the certificate is not found in the revocation list, it is certain that the certificate has not been revoked and the transaction is consulated without further action. If however, the certificate is found in the revocation list, it is highly suspect, but it is not conclusively certain that the certificate has been revoked. There is a small probability, a, that the certificate that was in the revocation list is in fact not revoked and would not be found in the master revocation list on the server computer. Thus, in order to provide the best possible customer service and be absolutely certain that a certificate that the client system indicates is revoked is in fact revoked, the client system must at this point go on line and inquire of the host as to whether that certificate in question is in fact revoked.

At this point in the process, the client and server are on line with each other. The communication link has been established and the attendant expenses associated therewith have been incurred. Thus another important aspect of this invention is to include the step of updating the locally cached revocation lists at this time. That is, when ever a certificate is found to be in the locally cached revocation list, the client will interrogate the server as to its validity. Then without breaking the communication link, the client will request and updated revocation list and the server will provide it.

The client may not make such a request with each interrogation. For example, if the highly unlikely event occurred that two inquiries were made within days or hours of each other, then the client may well forego receiving an updated list since its current one is only a few minutes or hours old. But the key element is that the client, not the server, makes the decision as to whether to receive an updated list.

A method has just been described for locally caching digital certificate revocation lists and updating such revocation lists based on the economics of the client rather than the server. The preferred embodiments just described are subject to numerous adaptations and modifications without departing from the concept of the invention. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Computation of Bloom Vectors

FIGS. 12a through 12c illustrate the bloom filter matrix calculation and build process 1006 which is performed in the server at some interval. As previously discussed, this interval may include when a threshold number of invalid keys are added to database 222, on a daily basis according to some predetermined scheduling algorithm, or any other implementation-specific interval. Upon the determination that a new bloom filter matrix should be created in the server, process 1006 starts at step 1202 to retrieve the range of parameters for which the bloom filter matrix will be created. These include a maximum value for M, M_(MAX), a maximum value for k, k_(MAX), the number of invalid keys N in database 222, and t, the number of bytes contained within each key. This also may include step sizes such as STEP_(M), STEP_(k) which define the necessary steps sizes between parameters supported by the server.

Then, at step 1204 the date and serial number which will be associated with the current bloom filter strings stored in the bloom filter matrix can be stored at step 1204. These are used by the client to determine whether the bloom vector string contained within a client 210 is current. Once these values have been stored, computation of the bloom filter matrix for the various parameters can commence.

At step 1206 an index m which is used for varying M for the matrix is initialized at step 1206. Step 1208 determines the p_(M) value for m using a known technique. Then, at steps 1210 and 1212, the indices k and j are initialized to 1. At step 1214, the coefficients p_(mkj) are computed for each of the varying values. These can be stored in a three-dimensional matrix which is addressable by specified values of m, k and j. In addition, an a value will be stored addressable by two parameters in a specified location in the matrix a_(mk) wherein a string can also then be retrieved based upon its a value. At any rate, at step 1216, it is determined whether all of the coefficients p_(mkj) have been calculated. That is, that j=k. If not, then j is incremented at step 1218, and steps 1214-1218 iteratively continue until such time as j=k as detected at step 1216.

Subsequent to the computation of the coefficients p_(mkj) of the hash functions for the various m and k values, the a value is then computed and stored with the associated m and k values at step 1220. Subsequently, at step 1222, it is determined whether k=k_(MAX). If not, k is incremented at step 1224 by STEP_(k). Steps 1212 through 1224 are then performed iteratively until it is determined at step 1222 that k=k_(MAX).

Subsequent to the determination that all coefficients have been now calculated for the current m value, at step 1226, it is determined whether all possible values of m have been used to calculate the coefficients stored in the matrix. That is, that m=M_(MAX). If not, then at step 1228, m is incremented by STEP_(m). Process 1006 returns to step 1208, and iteratively performs steps 1208-1228 until such time as all the possible values of m have been used for calculating the coefficients p_(mk), the prime key ^(p) M_(m) and a_(mk).

Upon completion of step 1226 and detection that the entire range of values of m have been used for calculation of the coefficients p_(mkj), process 1006 continues on FIG. 12b. FIGS. 12b and 12c illustrate the portion of the process which is used for calculating the bloom vector B from each of the keys contained in invalidity database 222. At steps 1230 and 1232, the indices m and k are initialized to 1. At step 1234, the bloom vector B_(mk) is initialized to zero. Then, at step 1236, the first key stored in invalidity database 222 is retrieved. Then, at step 1238, the index i and Q_(TOT) are initialized to 1.

At step 1240, the product of the t bytes ^(q) w_(i) of the key Q_(w) are accumulated in Q_(TOT). It is then determined at step 1242 whether i is equal to t, that is, the product of all of the bytes in the key Q_(w) have been accumulated in Q_(TOT). If not, as detected at step 1242, then the index i is incremented at step 1244. Steps 1240-1244 continue iteratively until the product of all of the t bytes ^(q) w_(i) have been accumulated in Q_(TOT).

Once the product of all the t bytes ^(q) w_(i) in the key Q_(w) have been accumulated in Q_(TOT), then at step 1246 a modulo of Q_(TOT) is performed using the prime key ^(p) M_(m). Once this has been performed, Q_(TOT) is used for computation of the bloom vector B_(mk). At step 1248, the index j is again initialized to 1. Then, the bloom vector B_(mk) is set equal to a bitwise OR function (`|`) of the current B_(mk) value and the product of the coefficient p_(mkj) multiplied by the Q_(TOT) value at step 1250. It is then determined at step 1252 whether j=k that is, that all coefficients p_(mkj) for the current m and k values have been hashed with the key and stored in bloom vector B_(mk). If not, then step 1252 proceeds to step 1254 to increment j, and steps 1250-1254 are iteratively performed until j=k, as detected at step 1252.

Once the bloom vector B_(mk) for the current m and k values has been computed based on all the coefficients p_(mkj) and this invalid key, then process 1006 proceeds to step 1256 of FIG. 12c. Step 1256 of FIG. 12c determines whether there are any more keys contained in invalidity database 222. If so, then the next key is retrieved at step 1258, and the process proceeds back to step 1238 of FIG. 12b. If not, then process 1006 proceeds to step 1260.

Step 1260 determines whether k=k_(MAX), the maximum k which will be supported by server 220. If not, then 1260 proceeds step 1262 to increment k and returns to step 1234 of FIG. 12b. Further processing for the next supported k is then performed for the bloom filter matrix.

Finally, if step 1260 determines that all possible k's for the current m value have been used for computation of the bloom vector, then process 1006 proceeds to step 1264 on FIG. 12c. It is then determined whether m=M_(MAX), the maximum possible M value. If not, m is incremented by the step size STEP_(m) at step 1266, and process 1006 proceeds back to step 1232 for computation of the bloom vector for all possible values of k for this value of m. Once it is determined that m=M_(MAX) at step 1264, then computation of the bloom filter matrix is complete, and it can be made available to any clients which may connect with the server. Once the calculation of the matrix is complete, accessing of a bloom filter string 400, is performed by accessing the values stored for client-specified and/or default M and k values (or a, if specified) and transmitted in a single string to a requesting client.

Thus, a method and apparatus of determining membership in a database, such as an invalidity database 222, has been described. This may be done, in some embodiments, without referencing that external database using a local invalidity cache implemented as a parameterized bloom filter stored in a client system. Such a system ensures that if a client determines, by use of the bloom filter, that the key is not a member of the database, it is guaranteed that the key is not a member of the database. Thus, no communication with a server and corresponding search need be performed. This thus conserves communication resources between servers and clients, and further guarantees that the key is not invalid. Although this invention has been described specifically with references to certain specific embodiments contained in FIGS. 2-12c, it can be appreciated by one skilled in the art that many modifications may be made without departing from the overall spirit and scope of this invention. Thus, the invention is to be construed by the claims which follow.

In early electronic certificate systems, the creator or the certificates pushes the revocation list to the users. That is, the creator of the certificates supplies the revocation list at its own discretion. The user/client has no voice in determining how often it may receive updated revocation lists.

For example, in the old credit card environment, a credit card was valid if (1) it had not expired and (2) it was not on the credit card issuer's revocation list. Credit cards that became invalid because they had expired could easily be checked because the expiration date is on the credit card. However the revocation lists created a different problem since it changed continually. As a partial answer to this problem, a written list of revoked credit cards was mailed periodically (weekly, monthly) to each merchant. If a credit card were invalid, the credit card company would not honor a purchase made with that card. The merchant had the responsibility of checking the list before consummating each sale.

The current practice among sophisticated merchants is to go on line with each transaction to check the revocation list. That is in the typical case, prior to consummating each transaction, a client computer with a modem makes a telephone line connection with a server/host computer and checks the invalidity data base resident in the server. There is only one revocation list; it is on line; and it is always current. The push of a revocation list from the issuer to the merchant is not necessary. This system is of course works well but it is expensive. And because it is expensive, it will not be suitable for many electronic transactions of the future, for example the purchase of a soft drink from a vending machine.

Thus, with the present invention, however, it is assumed that on line access to a current revocation list is not economically feasible for each transaction. However, it is economic to go in line occasionally. In this environment, revocation lists in the form of parameterized Bloom filters are stored locally in the client. The client can now decide based on its own economics when it wishes to have its revocation list updated. Thus, the revocation list may now be "pulled" from the server to the client. The factors that will most likely determine the frequency of such a pull by the client is the communication link connection cost, the risk and expense of turning away a valid customer and the value of the transaction.

With the present invention, however, a particular situation is set up where the pulling of an updated revocation list is likely to be economic and therefore desirable. With each transaction at the client, the revocation list is interrogated by hashing. If the certificate is not found in the revocation list, it is certain that the certificate has not been revoked and the transaction is consulated without further action. If however, the certificate is found in the revocation list, it is highly suspect, but it is not conclusively certain that the certificate has been revoked. There is a small probability, a, that the certificate that was in the revocation list is in fact not revoked and would not be found in the master revocation list on the server computer. Thus, in order to provide the best possible customer service and be absolutely certain that a certificate that the client system indicates is revoked is in fact revoked, the client system must at this point go on line and inquire of the host as to whether that certificate in question is in fact revoked.

At this point in the process, the client and server are on line with each other. The communication link has been established and the attendant expenses associated therewith have been incurred. Thus another important aspect of this invention is to include the step of updating the locally cached revocation lists at this time. That is, when ever a certificate is found to be in the locally cached revocation list, the client will interrogate the server as to its validity. Then without breaking the communication link, the client will request and updated revocation list and the server will provide it.

The client may not make such a request with each interrogation. For example, if the highly unlikely event occurred that two inquiries were made within days or hours of each other, then the client may well forego receiving an updated list since its current one is only a few minutes or hours old. But the key element is that the client, not the server, makes the decision as to whether to receive an updated list.

A method has just been described for locally caching digital certificate revocation lists and updating such revocation lists based on the economics of the client rather than the server. The preferred embodiments just described are subject to numerous adaptations and modifications without departing from the concept of the invention. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described herein. 

What is claimed is:
 1. In a method for updating a locally cached revocation list in a client computer with a database of revoked certificates stored in a host computer, the method comprising:establishing a communication link by the client computer with the host computer based upon circumstances defined at the client computer; and requesting by the client computer of an updated revocation list of digital certificates from the host computer, the updated revocation list being generated by a bloom filter having parameters selectable by the client computer.
 2. The method of claim 1, wherein the circumstances at the client computer comprising:upon receipt of a digital certificate submitted to the client computer for verification, checking by the client computer of the submitted certificate against the locally cached revocation list; if the submitted digital certificate is not on the locally cached revocation list, confirming the validity of the submitted digital certificate; if the submitted digital certificate is on the revocation list, making a communication link with the host computer and determining if the submitted digital certificate is in the database of revoked certificates; and requesting by the client computer that an updated revocation list be sent to the client computer.
 3. The method of claim 1 further comprising receiving the parameterized bloom filter by the client computer.
 4. The method of claim 1 further comprising updating memory of the client computer with the bloom filter.
 5. A method comprising:(a) initiating a communication link by a client computer with a host computer, the host computer contains a database of revoked digital certificates; (b) establishing the communication link; and (c) requesting by the client computer of an updated revocation list of digital certificates, from the host computer, said updated revocation list being generated by a bloom filter having parameters selected by said client computer.
 6. The method of claim 5 wherein the client computer contains a revocation list.
 7. The method of claim 6 wherein the revocation list is locally cached in the client computer.
 8. The method of claim 5, wherein prior to initiating the communication link, the method further comprisesreceiving a digital certificate by the client computer; and checking whether the digital certificate is listed in a current revocation list contained in the client computer.
 9. The method of claim 8, wherein prior to initiating the communication link, the method comprises determining whether the digital certificate is contained in the database of revoked digital certificates.
 10. The method of claim 8, wherein prior to initiating the communication link, the method further comprises confirming the validity of the digital certificate if the digital certificate is not listed on the current revocation list.
 11. The method of claim 10, wherein prior to initiating the communication link, the method further comprisesdetermining whether a bloom vector string associated with the parameterized bloom filter is being updated within the time interval set by the client computer, halting the establishment of communication link if revocation lists has recently been updated.
 12. The method of claim 5 further comprising loading the updated revocation list into the client computer.
 13. A method comprising:(a) checking a digital certificate submitted to a client computer against a revocation list stored in the client computer, the revocation list being generated by a bloom filter having parameters selectable by the client computer; (b) if the submitted digital certificate is on the revocation list, initiating a communication link by the client computer with a host computer, the host computer contains a database of revoked digital certificates and the client computer contains a current revocation list; (c) establishing the communication link; (d) requesting by the client computer of the current revocation list from the host computer; and (e) loading the current revocation list into the client computer.
 14. The method of claim 13, wherein the revocation list is locally cached in the client computer.
 15. The method of claim 13, wherein prior to checking the digital certificate, the method further comprises receiving the digital certificate by the client computer.
 16. The method of claim 15, wherein prior to initiating the communication link, the method further comprises confirming validity of the submitted digital certificate if the submitted digital certificate is not listed on the revocation list.
 17. The method of claim 15, wherein prior to requesting of the current revocation list, the method comprises determining whether the digital certificate is contained in the database of revoked digital certificates.
 18. In a method for updating a locally cached revocation list of digital certificates in a client computer with a database of revoked certificates stored in a host computer, the method comprising:receiving a digital certificate submitted to the client computer for verification; checking the submitted digital certificate against the locally cached revocation list by the client computer; if the submitted digital certificate is not on the locally cached revocation list, confirming validity of the submitted digital certificate; if the submitted digital certificate is on the revocation list, initiating a communication link by the client computer with the host computer; and requesting by the client computer that an updated revocation list be sent to the client computer.
 19. An apparatus comprising:a memory containing a condensed revocation list being a value formed by applying at least one hash function to a plurality of keys; and circuitry for retrieving an updated version of the condensed revocation list, the updated version is generated by parameters selected by a source remotely located from the apparatus, the parameters are associated with a bloom filter.
 20. The apparatus of claim 19, wherein the remotely located source is a client computer.
 21. The apparatus of claim 19, wherein the circuitry includesa module for requesting a remote server to transmit an updated version of the condensed revocation list; and a control circuit to control activation of the module.
 22. The apparatus of claim 19, wherein the plurality of keys used to form the condensed revocation list include at least one key deemed to be invalid.
 23. The apparatus of claim 22, wherein the memory is a local invalidity cache containing the bloom filter associated with the invalid key.
 24. The apparatus of claim 22, wherein the key is a unique value including a digital certificate.
 25. The apparatus of claim 22 further comprising:a client computer that includes the memory and the circuitry, the client computer to communicate with a host computer storing a database including a non-condensed list of invalid keys.
 26. The method of claim 22, wherein the key is a unique value including one of a credit card, a debit card number, an identification badge and a digitized biometric characteristic.
 27. A computer comprising:a memory containing a database including a list of revoked keys; a lookup module in communication with the memory, the lookup module accessing the database in response to a lookup request inquiring whether a selected key has been revoked; a control module in communication with the lookup module, the control module controlling the access of the database and responding to the lookup request by providing information whether the selected key has been revoked; and circuitry for generating a condensed revocation list being a value formed by applying at least one hash function to a plurality of keys, the circuitry comprisesa bloom filter matrix, which contained bloom filters that were generated from parameters selectable by the client computer, coupled to the control module, and a bloom vector calculator coupled to the bloom filter matrix and the memory.
 28. The computer of claim 27, wherein the control module further includes circuitry for controlling the bloom filter matrix to respond to an update request for an updated condensed revocation list. 